Revert "Theme: Promote ThemeProvider to stable API (#78958)"

[simison]

This reverts commit 3fb683e (PR #78958)

What?

Closes

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

Use of AI Tools

[github-actions]

Size Change: +1.19 kB (+0.02%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/modules/boot/index.min.js	51.1 kB	+49 B (+0.1%)
build/modules/content-types/index.min.js	160 kB	+179 B (+0.11%)
build/scripts/block-directory/index.min.js	44 kB	+200 B (+0.46%)
build/scripts/block-editor/index.min.js	383 kB	+118 B (+0.03%)
build/scripts/customize-widgets/index.min.js	14.6 kB	-5 B (-0.03%)
build/scripts/edit-post/index.min.js	52.2 kB	+42 B (+0.08%)
build/scripts/edit-site/index.min.js	298 kB	+189 B (+0.06%)
build/scripts/edit-widgets/index.min.js	22.2 kB	+1 B (0%)
build/scripts/editor/index.min.js	476 kB	+157 B (+0.03%)
build/scripts/format-library/index.min.js	29.5 kB	+9 B (+0.03%)
build/scripts/media-utils/index.min.js	115 kB	+58 B (+0.05%)
build/scripts/theme/index.min.js	22.3 kB	+190 B (+0.86%)

_{compressed-size-action}

[ciampo]

Closing in favour of #79594